How to avoid rapid actuator movements in favor of smooth movements in a continuous space and action space problem?
After some research on the subject, I found a possible solution to my problem of high frequency oscillations in continuous control using DDPG:

I added a reward component based on the actuator movement, i. e. the delta of actions from one step to the next.

Excessive action changes are punished now and this could mitigate the tendency to oscillate. The solution is nnot really perfect, but it works for the moment.

This finding is detailed out in the "Reward Engineering" section of my master's thesis. Please have a look into https://github.com/opt12/Markov-Pilot/tree/master/thesis 

I'll be glad to get feedback on it. And I'll be glad to hear better solutions than adding a delta-punishment.

Regards, Felix